Single page Print

Power consumption and efficiency
Our Extech 380803 power meter has the ability to log data, so we can capture power use over a span of time. The meter reads power use at the wall socket, so it incorporates power use from the entire system—the CPU, motherboard, memory, graphics solution, hard drives, and anything else plugged into the power supply unit. (We plugged the computer monitor into a separate outlet, though.) We measured how each of our test systems used power across a set time period, during which time we ran Cinebench's multithreaded rendering test.

All of the systems had their power management features (such as SpeedStep and Cool'n'Quiet) enabled during these tests via Windows Vista's "Balanced" power options profile.

Although we don't usually include "simulated" CPU speed grades in our power results, we've made an exception for the Q8200 out of sheer curiosity.

You can see detailed results in our Socket AM3 Phenom II review, but for this article, we decided to look only at the amount of energy used to render the scene. Since the different systems completed the render at different speeds, we isolated the render period for each system. We then computed the amount of energy used by each system to render the scene. This method should account for both power use and, to some degree, performance, because shorter render times may lead to less energy consumption.

In effect, we'll be looking at power efficiency per dollar. The 1/microjoules value in our power efficiency per dollar graph is really 1/(watt-seconds/1000000), or 1,000,000 m-2 kg-1 s2. That's a little obscure, but it quantifies power efficiency in a readable fashion based on the source data, which is in joules.

Since the cheap triple- and quad-core processors completed the test more quickly than the dual-cores, we're not surprised to see these CPUs score better on our efficiency-per-dollar chart. The Core i7 does deliver the best energy efficiency overall, which leaves the i7-920 in an enviable position on our value scatter plot. You'll pay dearly for the additional efficiency of the higher end Core i7 processors, though.